1. Field of the Invention
The present invention relates to a battery electrode substrate to be used for a positive electrode of an alkaline secondary battery or the like, a battery electrode formed from the battery electrode substrate, and an alkaline secondary battery including the battery electrode.
2. Description of the Related Art
In recent years, the use of the alkaline secondary battery for apparatuses, e.g., hybrid cars, power tools, and the like, which are required to have high powers, has been increased, and requirements for further increase in the capacity has become intensified. A nickel hydroxide electrode has been widely used as the positive electrode of the alkaline secondary battery, and has a structure in which a battery electrode substrate is allowed to support a positive electrode active substance for inducing a cell reaction.
For the battery electrode substrate, a sintered nickel plate formed by sintering a nickel powder, a punched nickel plate, and the like have been widely used. However, in this battery electrode substrate, it is difficult to increase the filling density of the active substance and, as a result, there is a problem in that the capacity of the battery cannot be increased.
Therefore, a battery electrode substrate composed of a three-dimensional network structure having a large porosity is expected as a battery electrode substrate capable of increasing the filling density of the active substance. For example, a three-dimensionally foamed nickel produced by subjecting a porous network structure, e.g., a foamed urethane resin or an unwoven fabric, to nickel plating and, thereafter, firing and removing the urethane or the unwoven fabric fibers is used.
However, the three-dimensionally foamed nickel has small mechanical strength and poor flexibility. As a result, when an active substance is filled by injecting an active substance synthesized paste from the surface of the battery electrode substrate, if the pressure of injection is increased for the purpose of increasing the filling density, breakage and the like of the network structure occurs. On the other hand, if the pressure is low, there is a problem in that a high filling density is not obtained. In order to solve the problems, battery electrode substrates provided with an unwoven fabric and a nickel plating film disposed on the fibers constituting the unwoven fabric have been proposed in Japanese Unexamined Patent Application Publication No. 2001-313038, Japanese Unexamined Patent Application Publication No. 2003-109600, Japanese Unexamined Patent Application Publication No. 2005-347177, and the like.
However, it is difficult for even battery electrode substrates disclosed in the above-described patent documents to further increase the filling density of the active substance satisfactorily. Consequently, there is a problem in that it is difficult to further increase the capacity of the battery.